In the textile industry raw fibers are subjected to a carding process in order to open fiber bundles, remove tangles, and disperse them into a uniform web. The historical development of carding technology as well as the present day usages is described by W. Edwin Sauer in "Formed Fabrics Industry" Volume 5, Number 10, October, 1974, pages 38 to 44. Further background on carding technology and carding machines is provided by U.S. Pat. Nos. 1,015,202; 2,513,299; 2,703,439; and 3,654,667. As far as it is known the carding technology has not be utilized for the treatment of plastic sheet material. This is not surprising since one would have expected that the use of such procedures and machinery would shred or otherwise destroy the sheet like nature of the plastic feed material.
A variety of processes and equipment have been described heretofore in the art concerning the manufacture of thermoplastic sheets having patterned surfaces. These processes have involved such features as molding, perforating, pile formation and the like to obtain the desired patterns in the thermoplastic sheet products. Some of the representative patents in this art include U.S. Pat. Nos. 2,924,863; 3,027,595; 3,317,644; 3,399,425; and 3,696,183. Many of these processes have the disadvantages of requiring relatively expensive machinery and/or surface modification equipment which not only is expensive but which is incapable of being readily changed in order to vary the desired patterns. In other words such prior art processes lack the flexibility required to produce different patterns in the thermoplastic sheet material without incurring prohibitive expenses. It would be desirable therefore to have available a process for producing patterned thermoplastic sheets by the use of molding surfaces which are not only relatively inexpensive but which may also be readily modified to obtain varied patterns.
One object of the invention is to provide a novel process for producing patterned sheets of thermoplastic material.
Another object of the invention is to provide a high speed, continuous process for modifying thermoplastic sheet material to obtain patterned, decorative surfaces.
A further object of the present invention is to provide a process whereby carding technology and carding equipment can be effectively employed in the treatment of thermoplastic sheet material for the obtention of repetitive patterns.
A still further object is to provide a process for continuously subjecting polyvinyl chloride sheet to pressing between continuously rotating means covered with card clothing to obtain a non-perforated or perforated patterned polyvinyl chloride sheet product.
These and other objects of the present invention will become readily apparent from the ensuing description and illustrative embodiments.
In accordance with the present invention it has now been found that thermoplastic sheet material such as polyvinyl chloride sheeting can be patterned by utilizing procedures and equipment employed in the carding of fibers provided that the thermoplastic sheet feed material is maintained at a temperature above its softening point while being pressed by the cooperating card clothing surfaces having surface wires maintained at elevated temperatures. It is another important feature of the present invention that the thermoplastic sheet be maintained during processing on the rotating cylinder to which it is fed and not stripped off by the rotating cooperative means which is also covered by card clothing thereby enabling the pressed sheet to be cooled on the rotating cylinder. This is accomplished by having the wires on the two carding surfaces point in the same or different directions and by having the wires on the upper cylinder set between the spaces of the wires on the cooperating or bottom surface. The present invention does require that the surface wires on the carding means be kept at somewhat elevated temperatures. It will be understood however that each surface may be maintained at either the same or different temperatures. The foregoing features will be discussed below in greater detail.
The flexible sheeting material which may be processed in accordance with the process of this invention may be composed of such thermoplastic polymers as plasticized polyvinyl chloride, low or medium density polyethylene, polypropylene, ethylene-vinyl acetate copolymers, vinylidene chloride, polyamides, polyesters, thermoplastic polyurethanes, and similar polymeric and copolymeric materials. In some instances the thermoplastic sheeting material may be backed or laminated with a material such as a non-woven or knitted fabric which is capable of being stretched in two directions from its axis. The thickness of the feed thermoplastic material is not critical, although in general its thickness will vary from about 0.002 to 0.060 inch, and preferably from about 0.004 to 0.010 inch. It will be understood that the thermoplastic sheeting may be supplied to the carding equipment from a roll of thermoplastic film and preheated prior to compression. An alternative method would be the thermoplastic sheet may be fed directly from an extruder which then furnishes the necessary preheating. Obviously a combination of these procedures can also be employed. The preheating can be accomplished by utilizing hot air blowers, radiant heaters, or the like. For most purposes it will be necessary to preheat the thermoplastic sheet prior to compression to a temperature at which the thermoplastic material will become soft, i.e., its heating softening temperature. In many instances, the thermoplastic feed material is heated to near its liquidus temperature to destroy the memory imparted by prior processing. This temperature will of course vary depending upon the nature of the thermoplastic material. With polyvinyl chloride sheeting, for example, the heating softening temperature is about 385.degree. to 400.degree.F.
In carrying out the process of this invention it is important, as pointed out above, to have the surface wires on the carding surfaces at somewhat elevated temperatures. The exact temperatures employed will vary with the type of thermoplastic material being treated, but in general it has been found essential to maintain the surface wires of at least one of the carding means at a temperature approximately equivalent to or below the heat softening temperature of the thermoplastic material. The temperatures of the surface wires can range from about 110.degree. to 280.degree.F., preferably from about 140.degree. to 200.degree.F., when utilizing a thermoplastic polymer such as polyvinyl chloride. Although the temperature of the surface wires on both of the carding surfaces may be the same, they may also differ. In the latter situation, for example, the cylinder, roller or endless belt means which serves as the molding surface may be maintained at the higher temperature, whereas the surface wires on the rotating cooperative means may be lower. Thus, with polyvinyl chloride sheeting the surface wires on the rotating cylinder may be within the range of about 120.degree. to 280.degree.F. and the surface wires on the rotating cooperative means may have a temperature within the range of about 110.degree. to 280.degree.F.
After the preheated thermoplastic sheeting is compressed between the nip formed by the two rotating carding means to form the desired patterned surfaces and passes out of the contact or nip zone it is cooled to a temperature below the softening point of the thermoplastic material. This cooling step can be accomplished by blasts of cold air or by keeping the internal temperature of the top cylinder or roll at a temperature of about 120.degree. to 200.degree.F. or any conventional cooling means. The cooled compressed thermoplastic sheeting is then parted or separated from the rotating cylinder in a continuous manner by the use of a conventional take-off roll, doffer or brush roll, to reach under the wires to strip the thermoplastic sheet. The take-off roll may also be cooled internally.